This invention relates to microwave components and more particularly to waveguide to microstrip coupling structures.
Waveguide to microstrip transitions are used in a variety of applications, such as in low loss antenna feed structures, high Q microwave filters and duplexers, high power combining devices, etc. This type of guided wave transition combines the low loss properties of the waveguide, with the flexibility of microstrip circuits. The topology is governed by the particular application at hand. As a result, numerous designs have been reported in the literature.
Some configurations are based on a monopole probe, whereby part of the microstrip or stripline circuit board protrudes through an opening in the broad wall of the waveguide to support the monopole appropriately. Other configurations require the microstrip circuit to be in the E-plane of the waveguide. Improvements have been made to address resonance problems and offer more general design guidelines. One design uses an electrically small microstrip radiating element in the E-plane of the waveguide, such as a quasi-Yagi antenna. These microstrip structures are mounted inside the waveguide.
Other transitions are based on aperture coupling between the microstrip and waveguide. This type of transition has the advantage that it eliminates the need for specially shaped printed circuit boards inside the waveguide, and it is very tolerant to small errors in the position of the aperture with respect to the waveguide. Some problems associated with this approach are that the aperture introduces additional radiation loss, and that it tends to have a limited bandwidth. Analysis of small aperture coupling between the end-wall of a rectangular waveguide and microstrip shows that such coupling is very small, due to a severe wave impedance mismatch between the waveguide and the microstrip loaded aperture. A larger, resonant aperture together with short-circuited microstrip stub matching yields better coupling. However, impedance matching is achieved only over a very narrow bandwidth and the high Q resonant microstrip stub adds to radiation and conduction losses. Matching structures inside the waveguide such as an E-plane waveguide fin also offer a lower loss but relatively narrow band solution. The introduction of a patch resonator and an additional dielectric quarter wave transformer inside the waveguide greatly increases the bandwidth, but this adds to the complexity and also introduces additional loss.
Aperture coupled transitions do not require the support of a specially shaped printed circuit board inside the waveguide, and the performance may be relatively insensitive to the position of the aperture in the waveguide. Early attempts with simple rectangular apertures did not produce coupling levels of practical significance. Some improvements, such as the addition of a short-circuited microstrip stub or an E-plane waveguide fin yield better coupling, but only over a narrow bandwidth. Another problem is that a resonant microstrip stub introduces extra losses, and the electrically large rectangular aperture tends to produces more radiation loss.
U.S. Pat. No. 6,127,901 discloses a transition having a slot in the broad wall near the short-circuited end of a rectangular waveguide, including a tapering narrow dimension for matching to a microstrip over a wide frequency band via an aperture coupled arrangement with an open circuited microstrip stub.
There exists a need for a waveguide to microstrip transition that provides an improved matching structure, has wide band coupling, and uses a relatively small aperture to reduce losses.
A waveguide to microstrip T-junction includes a microstrip transmission line structure having a ground plane separated from a strip conductor by a dielectric layer, the ground plane defining an aperture; a waveguide channel having a conductive periphery being electrically coupled to the ground plane to provide a waveguide short circuit wall located at the end of the waveguide channel; at least one conducting ridge inside the waveguide channel; and an end of the ridge being electrically coupled with the ground plane.